The present invention relates to an improved water impervious rubber or plastic insulated power cable.
A typical water impervious rubber or plastic insulated power cable has a structure wherein a water impervious layer comprising a metal-plastic laminated tape, a hold tape layer, and a metal shield layer are sequentially formed on a cable core. The cable core has a conductor shield, a rubber or plastic insulation layer, and an insulation shield which are formed on a conductor. Crosslinked polyethylene, an ethylene-propylene rubber, and the like are mainly used as the insulation layer in water impervious rubber or plastic insulated power cables having such a structure. Laminated tapes comprising lead or a lead alloy and polyolefin are mainly used as plastic laminated tapes constituting water impervious layers since they have excellent flexibility and chemical resistance. In addition, semiconductive cloth tapes are generally used as hold tapes applied on water impervious layers. A semiconductive cloth tape is obtained by friction-treating a woven fabric with electrically conductive rubber. The warps and wefts of the woven fabric are, in general, cotton yarn, viscose rayon yarn, acetate yarn, vinylon yarn, nylon yarn, or polyester yarn.
In the manufacture of water impervious rubber or plastic insulated power cables having the above conventional structure, a laminated tape comprising a metal foil and a plastic film is longitudinally applied on the cable core. After a hold tape is wound on the laminated tape, heating is performed to bond the laminated tape to the cable core, thereby forming a water impervious layer. In this case, the adhesive strength between the laminated tape and the cable core is largely influenced by the temperature and time during heating. That is, the higher the heating temperature and heating time, the higher the adhesive strength obtained.
A water impervious rubber or plastic insulated power cable manufactured in the manner described above, however, has the following problems.
A semiconductive cloth tape normally used as a hold tape on a conventional water impervious layer has a memory rate of expansion and contraction of 0 to 15%. Therefore, when a water impervious rubber or plastic insulated power cable having a hold tape constituted by such a tape is subjected to repeated bending, the water impervious tape cracks along the edge of the hold tape layer.
When the cable is subjected to a heat cycle of 130.degree. C..revreaction.R.T. (Room Temperature) with power applied to the conductor, as the cable core expands or contracts, the water impervious layer comprising a metal-plastic laminated tape bonded on the core also expands or contracts. However, the hold tape on the water impervious layer, once expanded, cannot contract. Therefore, when other layers expand and then contract, the hold tape layer forms wrinkles. Such wrinkles in the hold tape layer cause damage to the water impervious layer formed within the hold tape layer and/or the metal shield layer formed around the hold tape layer, thereby significantly degrading the characteristics of the cable.
The memory rate of expansion and contraction, which represents the degree of contraction of a yarn is an index for stretchability of a fabric of the yarn, and is calculated in accordance with JIS (Japanese Industrial Standard) L 1090 by the following equation:
Memory rate of expansion and contraction A(%)=(a-b)/a.times.100
where,
a=hank (mm) in the case of applying a load of 2/1000 gf.times.20 number per 1d of the indicated denier and another load of 1/10 gf.times.20 number per 1d of the indicated denier; and PA0 b=hank (mm) in the case of applying a load of 2/1000 gf.times.20 number per 1d of the indicated denier.